1. Technical Field:
This disclosure relates in general to passive flow control device and in particular to a passive device for delaying boundary layer flow separation within a high velocity fluid.
2. Description of Related Art:
All jet aircraft require a propulsion system which diffuses the incoming air to certain speeds before passing it through the jet engine. For supersonic aircraft, this process often involves a mixed compression inlet which initiates a series of shock waves that reflect off of the inlet surfaces. Each shock reflection causes a shock-boundary layer interaction near the point of reflection. Each interaction may include a shock-induced separation which reduces the inlet pressure recovery and degrades performance. The separation may also cause blockage, thereby reducing the effective flowpath area to a value below the critical level required for operation. This leads to an unstart, and limits the operational range of the inlet.
Shock induced separation can be reduced by actively bleeding (removing) the boundary layer from the flowfield, which requires porous surfaces and tubes/plumbing beneath the surface. The complexity and weight associated with porous surfaces and tubes/plumbing can degrade mission performance of the jet aircraft.
Vane-type vortex generators submerged in the boundary layer (i.e., micro-vanes) can modify fluid flow, but the contact surface of the micro-vanes is so small that they have a high likelihood of detaching and creating a foreign object damage (“FOD”) hazard. Ramp-type vortex generators submerged in the boundary layer (i.e., micro-ramps) can be attached more securely than micro-vanes, but their aerodynamic performance is worse than that of micro-vanes. Indeed, many studies show them to be worse than nothing at all because they introduce shock waves with an orientation which further reduces pressure recovery. Also, each micro-ramp creates a vortex pair in a position and orientation such that they induce an upward velocity (upwash) on one another which elevates them off of the surface (i.e., vortex lift-off). This upward velocity away from the surface diminishes their effectiveness at redistributing the boundary layer energy toward the floor.